A heterojunction bipolar transistor is one in which the emitter, base and collector regions are made of different semiconducting materials. In contrast, a homojunction bipolar transistor is made of a single crystalline composition with the regions and junctions therebetween defined by variations in the doping type and concentrations. In particular, there has been much recent effort directed toward the fabrication of InP/GaInAs heterojunction bipolar transistors for inclusion as optical receivers or transmitters in InP/GaInAsP optoelectronic integrated circuits (OEICs). However, to date such heterojunction structures have been fabricated vertically, usually in some sort of mesa structure. Such a vertical structure makes it difficult to reduce parasitic effects, such as extrinsic base resistance and collector capacitance, and to planarize the mesa.
Thornton et al have recently disclosed a lateral bipolar heterojunction transistor in a technical article entitled "Unified planar process for fabricating heterojunction bipolar transistors and buried-heterostructure laser utilizing impurity-induced disordering" appearing in Applied Physics Letters, volume 53, 1988 at pages 2669-2671. In this device, a 0.1 .mu.m undoped GaAs layer is formed between two thicker p-type Al.sub.0.4 Ga.sub.0.6 As layers. A central base region is then masked and Si is thermally diffused at 850.degree. C. into the emitter and collector regions. Impurity-induced disordering causes the GaAs in the emitter and collector regions to convert to n-type AlGaAs while the thermal treatment simultaneously converts the GaAs in the base region to become p-type. However, this technique requires a large impurity density to accomplish the disordering and is further considered to be incompatible with optoelectronic fabrication technique.